The items -- about 20 shotgun rounds and nearly a dozen razor blades -- were found at Baldy Mesa Elementary by a school employee, according to the Snowline Joint Unified School District.
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The discovery was made before parents and students arrived. Children were taken to a multi-purpose room and their parents were notified, according to a statement from the school district.
Parents were encouraged to keep their kids home or to pick them up if they were already at school.
The school district tweeted a statement following the announcement.
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The school is working with the San Bernardino County Sheriff's Department to investigate.
Parents were worried considering the discovery was the latest in a string of incidents.
"This is the third time within one week that something like this has happened," a parent said. "Last week they said used ammunition was on the side of the street, and Monday or Tuesday, there was a knife with a note. I guess it was a threat."
Late in the afternoon, detectives were alerted to more ammunition found on the street in front of the school.
Detectives said they were from an incident last week.
The investigation is ongoing.